The antitumor gene therapy presents a vast application prospect in view of the fact that it can fundamentally correct the abnormal gene expression and the corresponding imbalance appeared during the genesis and growth of tumors. However, currently there exists a few severe challenges in this field. For example, under the circumstances of systematic medication, how to control the delivered therapeutic gene merely within the solid tumor. Or, how to realize the high-level, high-specific expression of the therapeutic gene in the whole tumor while no expression at all in other normal tissues so that the side effects can be eliminated or reduced. Such technical challenges restrict the practical application of the gene therapy in treatment of solid tumors, as safety and efficacy are extremely important factors that have to be taken into consideration in the research and development of gene therapeutic vectors for tumor treatment. Many solid tumors, including most of primary tumors such as mammary cancer and melanoma, have characteristic anoxic or necrotic areas [Patyar S, et. al. 2010, Journal of Biomedical Science 17:21; Li X, et, al. 2003, Cancer Gene Therapy 10:105-111]. The partial pressure of oxygen in tumors, as tissue-oxygen analysis of the tumor patients has shown, is 10-30 mmHg while the partial pressure of oxygen in other normal tissues is 24-66 mmHg. In addition, the tumor cells in the anoxic or necrotic areas are not sensitive to radiotherapy and chemotherapy, which means that the traditional radiotherapy and chemotherapy are unable to kill the tumor cells completely. This consequently lowers down the tumor patients' tolerance to these traditional therapeutic methods, and results in high frequency of recurrence and metastasis of the tumors.
In view of these facts, the development of delivery systems and expression patterns of the therapeutic gene targeting to the anoxic or necrotic areas of tumors has recently become a hot topic in the field of gene therapy of solid tumors. Some anaerobic or facultative anaerobic bacteria, such as Salmonella typhimurium, Clostridium and bifidobacterium, can selectively colonize in the anoxic areas of tumors after a systematic infection process, [Patyar S, et, al. 2010, Journal of Biomedical Science 17:21; Li X, et, al. 2003, Cancer Gene Therapy 10:105-111]; and they can also stimulate the immuno-antitumor activities of the body. [Lee C H, et, al. 2009, Journal of Immunotherapy 32:376-388; Chen G, et, al. 2009, Cancer Science 100:2437-2443; Avogadri F, et, al. 2005, Cancer Research 65:3920-3927; Liu T, et, al. 2010, Cancer Gene Therapy 17:97-108; Eisenstein T K, et, al. 2001, Microbes and Infection 3:1223-1231]. However, the proliferation of bacteria cannot stop the growth of malignant tumors alone. In order to enhance bacteria's antitumor performance, researchers tried to introduce Salmonella bearing specific gene or protein to the tumor tissue while keeping other normal tissues unaffected. Recently, a series of progressions have been achieved by researchers in adopting Salmonella typhimurium as the tumor-targeted vector in gene therapy. A variety of attempts have been made to manipulate the expression of the antitumor gene. Prokaryotic promoters such as arabinose-induced promoter and outer membrane protein C (ompC) promoter, eukaryotic constitutive promoters such as β-actin promoter and the radiation-induced promoter found in eukaryotic cells, have all been put into experiments. They all present significant antitumor effect [Ganai S et, al. 2009, British Journal of Cancer 101:1683-1691; Brader P, et, al, 2008, Clinical Cancer Research 14:2295-2302; Weth R, et, al. 2001, Cancer Gene Therapy 8:599-611; Loessner H, et, al. 2007, Cellular Microbiology 9:1529-1537; Nguyen V H, et, al. 2010, Cancer Research 70:18-23; Loeffler M, et, al. 2007, Proceedings of the National Academy of Sciences of the United States of America 104:12879-12883; Loeffler M, et, al. 2008, Journal of the National Cancer Institute 100:1113-1116; Loeffler M et, al. 2008, Cancer Gene Therapy 15:787-794; Loeffler M, et, al. 2009, Cancer Immunology Immunotherapy 58:769-775; Lee C H, et, al. 2004, Journal of Gene Medicine 6:1382-1393; Lee C H, et, al. 2005, Cancer Gene Therapy 12:175-184; King I, et, al. 2002, Human Gene Therapy 13:1225-1233; Li Y H, et, al. 2001, International Journal of Cancer 94:438-443]. However, problems still exist in the above mentioned expression systems. For example, some researches showed that, when the attenuated strain of a certain bacterium was adopted to treat tumor-bearing mice, not only did it aggregate in the tumor but it also colonized in normal tissues [Clairmont C, et, al. 2000, Journal of Infectious Diseases 181:1996-2002], which resulted in undesired expression of the target gene in normal tissues and consequently in vivo toxicity; When recombinant Salmonella was adopted to carry constitutive promoters and to initiate the downstream target gene, the said multiple-organ distribution pattern may engender certain negative effects (for example, toxic effect) and therefore undermine the specificity of gene therapy [King I, et, al 2002, Human Gene Therapy 13:1225-1233; Low K B, et, al. 1999, Nature Biotechnology 17:37-41]; in addition, the high level of heterologous protein expressed through the strong promoter probably leads to intracellular toxicity, which will cause heavy loss of plasmid during the infection process [Hautefort I, et, al. 2003, Applied and Environmental Microbiology 69: 7480-7491]. Other problems include poor in vivo stability of the plasmid vector expressed by the gene and the low level of in vivo expression of the target gene (i.e. protein expression could not be detected by conventional methods such as protein electrophoresis and western blot) [Weth R, et, al. 2001, Cancer Gene Therapy 8:599-611; Li Y H, et, al. 2001, International Juornal of Cancer 94:438-443; Pawelek J M, et, al. 1997, Cancer Research 57:4537-4544; Liu S C, et, al. 2002, Gene Therapy 9:291-306; Nemunaitis J, et, al. 2003, Cancer Gene Therapy 10:737-744].